1. Field of the Invention
The invention relates generally to the field of communications. More particularly, the invention relates to overcoming phase shifts and amplitude attenuations incurred when a signal is sent from a transmitter to a receiver.
2. Discussion of the Related Art
As a signal propagates, the strength of the signal is attenuated. FIG. 1 depicts this attenuation (dB) as a function of frequency (MHz) for a series of ten UTP-5 cable lengths ranging from 10 meters to 100 meters. It can be appreciated from the family of curves depicted in FIG. 1 that the attenuation of the signal increases with increasing frequency, as well as increasing length of propagation.
In addition to attenuated amplitude, as the signal passes through the conductor, a phase shift will be incurred. This phase shift (delay) can be represented in terms of an excessive phase value of negative magnitude. FIG. 2 depicts this excessive phase (degrees) as a function of frequency (MHz) for the same series of ten UTP-5 cable lengths from 10 meters to 100 meters. It can be appreciated from the family of curves depicted in FIG. 2 that the magnitude of the phase shift imposed upon the signal increases with increasing frequency, as well as increasing cable length.
The practical implications of these attenuation and phase shift phenomenon will be made clearer in FIGS. 3-5. The data depicted in FIGS. 3-5 takes the form of time domain representations of amplitude for two signals of different frequency (i.e., a high frequency signal superimposed on a low frequency signal).
FIG. 3 depicts MLT3 data for a one meter long CAT5 cable. It can be appreciated from the time domain data shown in FIG. 3 that the amplitude of the high and low frequency signals has not be significantly attenuated by the one meter long CAT5 cable. The maximum amplitude trace for both the high and low frequency signals is substantially the same. Further, it can be appreciated that the high and low frequency signals have not undergone a substantial phase shift.
FIG. 4 depicts MLT3 data for a 40 meter long CAT5 cable. The maximum amplitude is lower in FIG. 4 compared to FIG. 3 due to the increased length of the cable in the case of FIG. 4. It can be appreciated from the time domain data presented in FIG. 4 that the maximum amplitude of the high frequency signal has decreased to a greater degree compared to the maximum amplitude of the low frequency signal. Further, it can be appreciated that the high frequency signal has been phase shifted more than the low frequency signal.
FIG. 5 depicts MLT3 data for a 110 meter length CAT5 cable. It can be appreciated from the time domain data presented in FIG. 5 that even the low frequency signal has undergone substantial attenuation and phase shift.
It has been known to use an equalizer to compensate for the amplitude attenuation and phase shifting phenomenon. A previously known circuit to implement an equalizer is shown in FIG. 6.
Referring to FIG. 6, the circuit includes a resistor 610 bridging a node M1 and a node M2. The resistor 610 has a value of 2R. The circuit also includes a capacitor 620 bridging a node A and a node B. The capacitor 620 has a value of 1/2 C. The circuit includes an input transistor 630 and an input transistor 640. Together, these components define an equalizer cell 650. This circuit has been implemented in discrete and bipolar technologies. The basic purpose of this circuit is to provide a high pass filter response to the desired signal.
Still referring to FIG. 6, selecting the frequency threshold of the high pass filter response can be termed providing a zero. The frequency of the zero is determined by the reciprocal of the product of R and C (1/RC). The input voltage can be represented as EQU .upsilon..sub.in =.upsilon..sub.i.sup.+ -.upsilon..sub.i.sup.-
where .upsilon..sub.i.sup.+, .upsilon..sub.i.sup.- are the input signals (differential). The output voltage can be represented as EQU .upsilon..sub.out =.upsilon..sub.0.sup.+ -.upsilon..sub.0.sup.-.
where .upsilon..sub.0.sup.+, .upsilon..sub.0.sup.- are the output signals (differential). I.sub.0 is the bias current for the equalizer cell, g.sub.m is the transconductance of the input transistors, and i.sub.0.sup.+, i.sub. .sup.- are the output currents (differential).
The general transfer function for the circuit shown in FIG. 6 is given by ##EQU1## where S represents frequency. In this equation, the additional pole is introduced at ##EQU2## for bipolar technology. Since g.sub.m is relatively large, the pole can be moved to a much higher frequency than that of the zero. The Bode plot (log--log) of this general transfer function is shown in FIG. 7.
There are significant problems associated with the circuit shown in FIG. 6 when implemented with integrated circuit technology. Four problematic areas are discussed immediately below.
First, a single value of RC can not cover all cable attenuations and phase distortions. In a local area network (LAN), an equalizer needs to compensate for attenuation losses in different cable lengths. While the upper limit of cable length for a LAN is somewhat arbitrary, an upper limit of approximately 100 meters is typical. In such a network there can be a wide variety of cable lengths of from 0 to 100 meters. Further, while different categories of cable are expected to exhibit different characteristics, cables of the same category that are produced by different manufacturers can exhibit different characteristics.
Second, the low frequency gain, R.sub.0 /R, varies due to the process of manufacture. Specifically, the values of R.sub.0 and R can vary 5%, even 10%, due variations in materials and/or processing. Further, the low frequency gain also varies with the temperature of operation. As the circuit warms up, the values of R.sub.0 and R will change as a function of temperature.
Third, the location of the high pass response threshold (the zero), 1/RC, varies due to the process of manufacture and the temperature of operation. Further, the accuracy of the individual components can be affected by parasitic components at nodes A and B. Furthermore, the body effect of transistors M.sub.1 and M.sub.2 will cause the transfer function of the equalizer to deviate from the ideal characteristics.
Fourth, for high frequency operation, g.sub.m /C can be very close to the frequency where the signal is transmitting. This limits the maximum frequency of operation of the equalizer cell 650, which is nevertheless required to provide a gain at high frequencies. This is especially problematic for 100BT or 1000BT operations. In addition, most of the high-level integration data communication circuits are using CMOS technology. The transconductance of CMOS transistors is much lower than that of the bipolar counterpart. This creates a significant problem because the unwanted pole (g.sub.m /C) will be even closer to the zero frequency when the transconductance is low.